Conservative calculations show that the upper subducted slab (basal + sediment) passes beyond the arc magma genesis zone and enters the deep mantle with a minimum of 500-1000 ppm K, and corresponding amounts of other incompatible elements. If this material is not completely homogenized with the mantle and later becomes part of the source of ocean island magmas, then the ocean island arc isotopic similarity is a result of their similar mix of source materials-mantle peridotite with trace element signatures from oceanic crust and sediment.